zebra: skip blackhole nexthop when sending multipath netlink updates

[bhinin]

In case of multipath, when zebra is sending netlink updates to the kernel, if one of the nexthops is a blackhole zebra tags on unnecessary bytes at the end of the message.

This is rejected by kernel resulting in zebra marking the route as rejected. This change fixes that by skipping a blackhole nexthop

[ton31337]

Dup https://github.com/FRRouting/frr/pull/14606?

[bhinin]

Dup #14606?

No, this is a separate bug fix. I raised the pull request now for ease of discussion in #14606.

[bhinin]

This is not a rt_netlink.c problem. It's a generic problem that should be handled higher up in zebra before anything reaches the dataplane.

From a routing perspective it makes no sense ( and the linux kernel doesn't allow you ) to have a ecmp path that has 1 blackhole and other paths that can be sent along )

This should be fixed up in the nexthop resolution phase inside of zebra_nhg.c probably nexthop_active_check and the route should be marked as uninstallable.

Thanks!

rt_netlink is already adding the required space to the message and changing the length. It's just when it's time to add the ifindex that it checks for NEXTHOP_TYPE_BLACKHOLE. and skips. This results in some 0s tagged at the end of the netlink update.

Are you suggesting that this be fixed such that the blackhole nexthop would not even show up when ALL_NEXTHOPS_PTR(dplane_ctx_get_ng(ctx), nexthop) is called?

[donaldsharp]

yes exactly. This is not a dataplane problem as that the same thing can happen on non-linux or even fully asic platforms. As such it needs to be handled before it hits the data plane code in any way shape form or fashion

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